Distiller's grains are a byproduct from brewers and ethanol plants. Wet distiller grains are a primary byproduct of the 1st generation corn ethanol industry. In a dry mill process, the entire grain kernel is ground (milled) into flour. The flour is physically and chemically prepared for fermentation via cooking to produce a mash and the mash is hydrolyzed to release the sugars using enzymatic saccharification. The sugar mixture, with solids, is then fermented to produce ethanol. The fermented product is distilled to separate ethanol from the unconverted solids and process water as stillage from the bottom of the distillation tank. In a typical process whole stillage is separated into thin stillage (liquid fraction) and a solid fraction known as wet distillers grain.
Wet distillers grains (WDG) contains primarily unfermented grain residues (protein, fiber, fat) and up to 70 percent moisture. WDG has a short shelf life (a week or less) and transportation involves movement of high water weight. WDG supply transport is economically viable usually within a 200 km radius from the ethanol production facility. These facts are important as they affect both profitability and logistic issues. WDG can be dried to produce Dry Distiller Grains which can be optionally combined with concentrated thin stillage (heavy syrup) to produce dried distillers grains with solubles (DDGS). DDGS is an animal feed that has an almost indefinite shelf life and may be sold and shipped to any market. Drying is costly, both from a capital and energy point of view, according to a recent article (Ryan C. Christiansen “DDGS: Supplying Demand” Ethanol Producer Magazine, March 2009) up to 30% of the operating budget in a modern first generation dry-grind ethanol facility being used to produce dry distiller grains with solubles. As a result this commodity product represents little overall value to the corn mill. WDG contains sugars in the form of unconverted polysaccherides such as cellulose, hemicellulose, and starch within the fibers. Thus conversion of these latent sugars into additional ethanol represents a value added stream for a 1st generation ethanol plant.
A primary obstacle to the usage of the fiber in WDG is an expensive “pretreatment” step to release these sugars and make the cellulose and other polysaccharides in the feedstock accessible to enzymatic hydrolysis. One of the leading candidates for such a pretreatment is dilute mineral acid hydrolysis (typically sulfuric or hydrochloric acid). The conditions of a successful pretreatment in dilute acid hydrolysis are determined by a combination of three factors; time, temperature, and acid concentration. Increased temperatures lead to loss of sugars to degradation products and increasing acid concentration (to lower the temperature) comes at the expense of the acid employed and neutralized salts in downstream equipment.